The issue with IPOs as the marker of "success" is that public market investors are much more hostile to companies without the VC stamp of approval - they're relying on the implicit due diligence of the VC firms to make sure that there are no issues with the technology, the patent landscape, and other various hangups. (This applies much more to health care companies, where my VC experience was, but the mentality is the same as the public market investors are the same regardless of industry.)